The present invention relates to a gas permeable porous resin film and a process for producing the same. More particularly, the present invention relates to a polyolefin resin porous film produced by film-forming a mixture of a polyolefin resin with a thermoplastic polyester resin and stretching the film and a process for producing the same.
Heretofore, there have been known various methods for the preparation of a gas permeable porous resin film, including (1) a method wherein a non-porous film is perforated by an electro-discharge treatment, (2) a method wherein a resin containing calcium carbonate particles is formed into a film and then the calcium carbonate particles are dissolved away from the film by an acid treatment to give a porous film, (3) a resin containing particles of an inorganic substance such as calcium carbonate is formed into a film and then the film is stretched uniaxially or biaxially, and (4) two kinds of resins which are less compatible to each other are blended and formed into a film and then the film is stretched uniaxially or biaxially. These methods have been used to produce a water vapor permeable film or a gas permeable film. The method (1) and the method (2) among the above-mentioned methods are undesirable from the standpoint of productivity and the method (3) is widely used.
However, a porous film obtained by stretching a film containing particles of an inorganic substance according to the method (3) is liable to have a high rigidity due to a high rigidity of the filler contained. Further, when the film is crumpled with hands, the filler particles tend to fall off, which causes an uncomfortableness in the use thereof and further become a dust to damage an environmental condition depending upon use of the film. Of course, in the process of the preparation of the film, the environmental condition tends to be worsened due to the dust of the inorganic minute particles. Moreover, a porous film containing particles of calcium carbonate which is generally used as the inorganic filler has a drawback that the water proof thereof is lowered with the lapse of time due to the hydrophilic property of calcium carbonate.
The method (4) in which two kinds of resins which are less compatible with each other are blended and formed into a film, which is proposed in Japanese Examined Patent Publication No. 53-12542 and Japanese Unexamined Patent Publication No. 58-198536, has various drawbacks mentioned below.
The former (Japanese Examined Patent Publication No. 53-12542) discloses a method wherein a mixture of polypropylene with polystyrene, the amount of which is from 0.05 to 10% by weight per the amount of polypropylene, is formed into a film and the film is stretched to obtain a film having voids. However, these voids are not a through hole penetrating through the film along its thickness direction as described in the publication, and therefore the method is not one for producing a gas permeable or water vapor permeable film. When the amount of polystyrene used is more than 10% by weight per the amount of polypropylene, the resulting stretched film has voids penetrating through the film. However, the number of the voids is small and the stretching of the film is difficult.
Further, in the case of a film obtained by a simple method wherein a polyolefin resin as a base resin and a polystyrene resin as an auxiliary resin are blended and the resulting blend is film-formed, the polystyrene resin is not finely divided so that it is difficult to stretch the film and pores formed by stretching the film are course. As a result, only a film having a very poor water proof is obtained. Even if the polystyrene resin is dispersed in a desirable particle form, the formed polystyrene particles are deformed unless the temperature for the film-forming is considered. As a result, the stretching property becomes poor and, even if the film can be stretched, the pore size of the pores formed in the stretched film is so large that the film is poor in water proof or voids are present discontinuously in layers along the thickness direction of the film and not continued so as to form pores penetrating the film, which results in formation of only a film having a poor gas permeability and a poor water vapor permeability.
The latter (Japanese Unexamined Patent Publication No. 58-198536) discloses a method wherein a mixture of a thermoplastic resin (a) and a thermoplastic resin (b) which is less compatible with the resin (a) and has a melting temperature of more than that of the resin (a) by 20.degree. C. or more is melted and kneaded at a temperature of more than the melting temperature of the resin (b) and thereafter film-formed at a temperature of more than the melting temperature of the resin (a) and less than the melting temperature of the resin (b), and the resulting film is stretched at a temperature of less than the melting temperature of the resin (a) to give a water vapor permeable film.
According to the method, the once formed particles of the resin (b) tend to again agglomerate, thereby yielding large particles, so that it is difficult to obtain a film having a good water proof.
An example of the above-mentioned method wherein polyethylene resin or polypropylene resin is used as the resin (a) and a thermoplastic resin such as polyethylene terephthalate or polybutylene terephthalate, which is usually used for the formation of films, is used as the resin (b) will be explained. The usual film stretching temperature is from 60.degree. to 100.degree. C. for a polyethylene film and from 60.degree. to 110.degree. C. for a polypropylene film. The glass transition temperatures of polyethylene terephthalate film and polybutylene telephthalate film are from 60.degree. to 70.degree. C. and from 40.degree. to 60.degree. C., respectively. Even if a mixture of one of the above resins as the resin (a) and one of the above resins as the resin (b) is kneaded under heating and formed into a film wherein the resin (b) is dispersed to become particles having sizes within the optimum particle size range, the particles of the resin (b) are deformed and stretched during the stretching step, when the stretching is conducted at a temperature more than the glass transition temperature of the resin (b) in such a state that the resin (b) is not crystallized. As a result, there occur cases wherein pores are not formed and only a film having a poor gas permeability and a poor water vapor permeability is obtained.
In order to eliminate these drawbacks, an attempt wherein a film is stayed for some time after the film is formed, thereby promoting the crystallization of the resin (b), and thereafter the stretching is conducted has been made. However, such an attempt cannot improve the gas permeability to a sufficient degree, even though it is possible to improve these properties to some extent. Further, if polyester resin having a small molecular weight is used as the resin (b) in order to attain an easy crystallization of the resin (b), particles of the resin (b) having a large size are formed because the viscosity of the resin (b) becomes excessively low. When a film containing such large particles is stretched, pinholes having a diameter of more than 1 mm are liably formed and the film is apt to be broken. On the other hand, if a resin having a low molecular weight is used as the resin (a), the stretching property is lowered. Thus, both means are impractical.
In the case of biaxially stretching such a non-stretched film as mentioned above, it is necessary to stretch the film in a stretching ratio of not less than 1.5 in each of the longitudinal and transverse directions in order to obtain stably a uniform stretched film. When the stretching ratio is less than 1.5 in each of the longitudinal and transverse directions, non-stretched portions remain to give only an ununiform film having a striped or spotted pattern which results in a poor appearance. Further, there are some problems with respect to the stability of stretching. In particular, in the case of a tubular,biaxial stretching method, it is very difficult to stretch non-stretched film in a stable condition. Further, an ununiform stretching is an undesirable factor against obtaining a film having a desirable gas permeability.
When the stretching ratio is more than 1.5 in each of the longitudinal and transverse directions, it is possible to stretch a non-stretched film stably and uniformly. However, when the non-stretched film is stretched uniformly as described above, a tear strength of the stretched film in the longitudinal direction is remarkably decreased.
Thus, the prior art has a problem that when a non-stretched film is stretched in a low stretching ratio in order to obtain a stretched film having a sufficient tear strength, the stretching is ununiform and, on the other hand, when an attempt for obtaining a film having a good gas permeability by stretching an unstretched film in a high stretching ratio is made, the tear strength is decreased, which is inadequate for some use. There has been a demand for improvement on these points.
It is an object of the present invention to provide a process for producing a porous resin film having an excellent water proof and an excellent water vapor permeability.
Another object of the invention is to provide a porous resin film having an excellent water proof and an excellent water vapor permeability.
Further object of the invention is to provide a process for producing a porous film having a high tear strength in addition to the above-mentioned properties, and to provide the porous film produced by the process.
These and other objects of the present invention will become apparent from the description hereinafter.